Almost all gold currently produced from ores in the world is extracted by cyanidation leaching. Since its discovery in the 1890,s, cyanidation has been the method of choice for the extraction of gold and silver from their respective ores. This method of extraction, however, has several draw-backs. First, cyanide is an extremely toxic compound, creating the need for expensive transportation, storage and cleanup procedures. Many countries no longer allow the construction of new gold processing plants using cyanide. Furthermore, the leaching kinetics of gold and silver with cyanide is very slow. Leaching residence time is typically 2-4 days.
In addition, the gold industry in the U.S.A. and the world is facing problems associated with refractory gold-bearing deposits. One of the major challenges currently experienced by the extractive metallurgy industry is the efficient recovery of precious metals from refractory ores. The term refractory ores is used to describe a family of ores bearing precious metals where the precious metals such as gold and silver tend to be locked in various host minerals, primarily sulfide minerals and other rocks. In other cases, gold and silver are associated with carbonaceous material. When gold and silver are leached into solutions from such carbonaceous ores, dissolved gold and silver ions are reabsorbed on the surface of carbonaceous material. As a result, the overall recovery of these precious metals is reduced due to the presence of this carbonaceous material. The processing of such an ore deposit is extremely difficult and costly, and consequently, numerous gold mines worldwide have abandoned their operations due to unfavorable economic ground.
Such refractory ores are common in gold mines. Typical examples include Carlin, Cortez, Gethell, Bald Muntain, McLaughlin, Blue Range, Jardine and Mercur Mines in the U.S.A., Campbell Red Lake, Giant Yellowknife and Ker Addison Mines in Canada, Sao Bento Mineracao in Brazil and Fairview Mines in South Africa.
There have been various ways practiced in industry to treat such refractory ores. Commonly practiced techniques for sulfide ores include high temperature roasting of the ore followed by leaching, and pressure oxidation in an autoclave before the extraction of these precious metals from the ore. Numerous mining operations in South Africa still use roasting followed by a leaching process, while a number of mines in the world use a process where the ore is subjected to oxidation in an autoclave at an elevated temperature followed by the conventional cyanidation process. The cost of this two-stage process is found to be excessive and, therefore, these companies have long been looking for alternative ways of treating this refractory ore. Furthermore, the recovery of silver from such a two-stage process is known to be poor.
There have been numerous attempts made to overcome such problems associated with refractory ore processing using various chemical reagents such as thiourea.sup.1,2, halogen chemicals.sup.3 and ammoniacal thiosulfate.sup.4. However, though these chemicals are effective in dissolving precious metals in solutions, due to high reagent consumption and/or poor selectivity, these reagents are not being adopted by the precious metal industry.
In the case of carbonaceous ores, the carbonaceous material is destroyed or treated before extraction of gold is attempted. The carbonaceous material is frequently subjected to oxidation using ozones, chlorine, sodium hypochlorite, permanganates, perchlorates, and oxygen. Chlorine is currently used in the treatment of these carbonaceous ores.sup.5. However, due to the high cost of chlorine, it is desirable to find an alternate way of treating this ore.
Accordingly, it is an object of the present invention to provide an improved process for the extraction of gold and silver from their ores, which improved process is substantially free of one or more disadvantages of prior processes.
Another object is to provide an improved process for the extraction of gold and silver from their ores, which process does not employ cyanide.
Still another object of the present invention is to provide an improved process for the extraction of gold and silver from their ores which gives a greater yield of gold and silver than does prior processes and which is less expensive, more economical, and safer.
Additional objects and advantages of the present invention will be apparent to those skilled in the art by reference to the following detailed description.
The current invention utilizes ammonia as the key solvent to dissolve gold and silver from their solid forms, such as native state or complex ores. Ammonia is non-toxic, relatively inexpensive and easily regenerated by evaporation. Leaching of copper, nickel and cobalt using ammonia/ammonium salts has been well established. One of the applicants has had years of experience with these systems and published extensively.sup.6-10. Thermodynamic studies on the gold-ammine systems have been done.sup.11,12. However, no successful attempts have been made to extract gold and silver from various ores or other materials using ammonia. The problem has been the slow rate of dissolution when proper oxidants and sufficiently high temperature and pressure are not applied. This new technique using ammonia for the extraction of gold and silver is effective not only for the conventional sources of gold and silver but also for the refractory ores in a one stage operation with excellent recovery of gold and silver.